Chips are processed in batches during the process for manufacturing semiconductor devices, in which a large number of complicated devices are formed on one piece of wafer. The size of chips becomes smaller and smaller while their integrating density grows higher and higher with speedy development of GSI (Grand Scale Integration).
SCE (Short Channel Effects) occurs when the MOS (Metal Oxide Semiconductor Transistor) channel is shortened to a certain degree. When the length of the channel is decreased to a certain degree, the depletion regions of source junction and drain junction have increased proportions in the whole channel, and the quantity of charge required for the formation of an inversion layer on silicon surface below the gate decreases, thus the threshold voltage (Vt) decreases, and the cutoff current (Ioff) increases at the same time. Owing to SCE, the threshold voltage becomes very susceptible to length change of the channel, which makes the control on the process for manufacturing semiconductor devices become more difficult.
An Ultra-Shallow Junction process is generally employed in 65 nm-below scale semiconductor process to suppress SCE of a CMOS (Complementary MOS) device. Moreover, low energy boron ion implantation process is used in Lightly Doped Drain (LDD) process for a PMOS (P-channel MOS) device. Therefore, in order to reduce the diffusion of boron atoms in silicon substrate and obtain ultra-shallow junctions, a carbon co-implantation process may be employed during the LDD implantation process. The carbon co-implantation process contributes to the formation of ultra-shallow junctions for the reason that carbon atoms help to reduce the diffusion of boron atoms in the silicon substrate.
However, since the polysilicon gate is carbon co-implanted during the LDD implantation process, and in the following performed P-type heavily doped boron implantation (P Plus Implantation) process and the thermal annealing process, carbon atoms implanted in the LDD implantation process reduce the diffusion of P-type heavily doped implanted boron atoms in the polysilicon gate as well, the boron atoms in the polysilicon gate can not diffuse sufficiently and the resistance of the P-type polysilicon gate increases.